1. Field of the Invention
The present invention generally relates to microstructures, and more particularly, microstructures having coated internal surfaces for facilitating chemical processing and/or manufacture and methods of coating same.
While the present invention is subject to a wide range of applications, it is particularly well suited for improving catalytic reactions within one or more microchannels of a microreactor.
2. Technical Background
Chemical reactors are widely used in industry to carry out reactions between two or more chemical components, for example, between liquids and liquids, gases and gases, slurries and slurries, liquids and gases, liquids and slurries, gases and slurries, solids and solids, solids and liquids, solids and gases, and solids and slurries. Many of these reactors are large, fixed-site units designed for continuous operation at roughly constant throughput. These reactors typically have a conventional shell-and-tube design wherein reactants pass through catalyst-containing tubes while heat, usually in the form of hot gases contained within the shell, is applied to the outside of the tube.
A major drawback to shell-and-tube type reactors in general is the size of the reactors themselves. Their relatively large size makes these reactors less desirable for use in applications requiring a more compact reactor. These and other conventional processing equipment also suffer from additional disadvantages. It has long been recognized in the chemical industry that “scale-up” from laboratory bench scale to commercial production scale is difficult. Results achieved in the laboratory are often difficult at production rates in production facilities. The conventional wisdom of “economy of scale” is based upon economic considerations which relate production rate (units of production per unit of time) to capital investment. This conventional approach results in less than optimum precision of control of chemical processing.
In recent years, these and other shortcomings have been largely overcome with the advent of microstructure/microreactor technology. Microreactors, i.e., structures having one or more microchannels through which fluids may be passed, processed, analyzed, and/or caused to react, although in their infancy, have been successfully developed and operated for homogeneous applications (applications where a plurality of reactants or the reactant(s) and catalyst(s) are in the same phase, for example, the liquid phase). Although microreactors and microreactor systems have been developed for heterogeneous applications, i.e., applications where the plurality of reactants or the reactant(s) and the catalyst(s) are in different phases, such microreactors have met with substantially less success.
Generally speaking, typical heterogeneous applications within microreactors involve the use of one or more catalyst(s). A catalyst increases the rate of a reaction without being consumed by it, and typically operates by lowering the activation energy for a chemical reaction. Most commonly, the preferred catalyst for use in microreactors is a solid catalyst that increases the rate of a fluid phase (gas, liquid, or gas-liquid) reaction. An optimum catalyst should have the preferred attributes of activity, selectivity, stability, and regenerability. Unfortunately, obtaining and retaining all of these catalyst attributes in the field of microreactors is a difficult task.
While microreactors have been manufactured from materials such as silicon, the vast majority of microreactors have been fabricated from metals, such as stainless steel. Accordingly, most of the research relating the use of catalysts in microreactors has been directed to the use of catalysts in metal microreactors. One widely used approach in such metal microreactors is to tightly pack a plurality of small solid particles of catalyst material within the microreactor microchannel, and thereafter flow the reactant(s) across the, “packed-bed.” The predominate drawback of such a technique is the pressure drop created within the microreactor when utilizing such an approach. Additionally, in multi-channel microreactors, or microreactors in parallel, differential drops in pressure will significantly effect performance. Moreover, stability of the packed-bed is an issue as rapid flow velocities within the microchannel(s) tend to displace the catalyst particles leading to catalyst attrition and then decreased performance, and the need for a filtration step.
As a result of these shortcomings, attempts have been made to coat the microchannel walls within metal microreactors. Suitable coating techniques have included dip coating, brushing, spraying, and sputtering. Due in particular, however, to the co-efficiency of thermal expansion differences between the metallic wall surfaces of such microreactors and the catalyst carrier, particularly alumina, it is extremely difficult with conventional coating technology to have strong coating adherence to the microchannel walls utilizing such approaches, as it is well known that the adhesion of typical catalyst carriers to metal is very poor. This is particularly true when coating thicknesses exceed several tens of microns.